The discovery of high temperature superconductors has lead to the development of a number of applications for their use. Superconductors are known to have the property that they have zero direct current (DC) resistance below a critical temperature Tc. They also have zero DC resistance below a critical current Ic and a critical magnetic field Bc.
One potential use of HTS is in FCLs. HTS can be used in FCLs in a number of ways, and the use of HTS to limit fault currents is an elegant solution to the ever-present short circuit threat in power networks.
Traditionally, electrical systems have been developed around three separate phases and a neutral path. Further, authorities often mandate that faults in the system should be dealt with in a controlled manner. In particular, there is a concern to limit the effects of fault currents within the electrical distribution system. The HTS FCL designs are ideal for limiting fault currents.
There are a number of different classes of faults which can occur on a three phase transmission system. These include:
1. Three Phase Faults
In this case, a short circuit is formed between all three phases of the three phase line. A typical example of this occurs when a tree branch falls directly across the three phases of an overhead line. In addition, this fault may occur between all three lines and the neutral conductor, such as when a cable, with a neutral connected as an earthed shield, is severed by excavation equipment. Both types of faults are also known in the industry as symmetrical faults, because the fault current in each of the three phases will be of the same steady state RMS magnitude. Under three phase short circuit conditions, and in a balanced three phase system, operating at a constant frequency (eg. 50 Hz) and sinusoidal currents and voltages, the neutral/earth fault current will be negligible in the steady state because all three fault currents will remain at a 120 degree phasor displacement, thus cancelling vectorially and leaving a null neutral/earth current. Hence, no devices are normally required in the earth/neutral circuit to protect against this type of fault.
2. Double Phase to Ground Faults.
In this example, a short circuit is formed between two phases of the network and ground. The resulting steady state fault current therefore does not cancel (in the steady state mode) as in (1), but adds vectorially to form an earth/neutral fault current.
3. Single Line to Ground Faults.
For single line to ground faults, a single line forms a short circuit directly to ground. The resulting fault current therefore flows through the ground back to the source neutral. An example of this occurs when an underground 3 phase cable is pieced by excavation equipment, or, when a single bare overhead line falls and touches the ground.
It would be desirable to reduce the deleterious earth fault currents for double phase to ground faults and single line to ground faults
Transient Features of Fault Currents:
The fault current waveform resulting from either of examples (1) to (3) will contain features in the time domain, at the instant of the fault occurring, which are referred to as fault transients. The typical current versus time shape of these fault current waveforms is shown in FIG. 1. The transient fault current portion 24 and the steady state fault current portion 23 are clearly shown. The fault level on a system is typically specified or calculated (for example when MVA is used instead of kA) only in terms of the steady state value 23 of the fault current. However, with modern switchgear, which can open in 2-3 cycles on very high voltage networks (above 230 kV for example) or 5-10 cycles on lower voltage systems (below 110 kV for example), it is the full transient as well as the steady state current which most plant will be subjected to during fault conditions.
The fault current waveform of FIG. 1 also shows a DC component which dies away steadily. Both the three phase fault current and the earth/neutral current will behave in this way, for example, in the latter case, for a single line to ground fault.
However, in particular, there is a need to limit not only the three phase fault currents but also earth fault currents which flow when there is a short of one or two of the phases to ground. Protection from the effects of such faults is normally provided by neutral earthing resistors from manufacturers such as Cressall. These resistors can cost in the vicinity of AU$250,000 to purchase and install and are a significant undertaking. The use of neutral earthing resistors has a number of problems. Firstly, they are only useful when the fault is to ground. Secondly, they increase the voltage stress on the other phases when a fault occurs. They often require extra insulation of the neutral, require extra expense for transformers and are not cost effective unless protection is also upgraded.
Another solution to rising fault levels at substations include upgrading the low voltage side switchgear. This option often requires a substantial investment in capital and labour and is only effective as long as the fault level remains below the fault level of the new switchgear. This is not always true because subsequent additional transformers, and/or a reduction in the substation source impedance, can lead to the future fault level increasing beyond the new switchgear rating. An alternative solution to upgrading the switchgear is to split the bus and transformers into a number of isolated circuits such that each part of the load is supplied from a reduced number of parallel transformers.
For example, if a substation has two transformers operating in parallel, and the fault level of the switchgear is exceeded, then the bus may be split into two separate circuits each supplied by a single transformer. This doubles the substation fault impedance, which is desirable because the fault level will be nominally halved. However, this gain is at the expense of reduced reliability. Operating on a split bus means that if one transformer fails, all the load (i.e customers) connected on that transformer will lose supply until they can be switched over to the remaining good transformer. This can take up to 10 seconds which is sufficient to shut down computers and other sensitive factory automation and control equipment. Hence, it is not desirable for a utility or electrical network owner to operate in the split bus mode.
A further solution is to install so called “series limiting reactors”, which act liked fixed value inductances and therefore present an impedance to the network and to fault currents. This technique, however, often leads to voltage regulation problems as the impedance also exists during normal operating conditions.
Further, higher impedance transformers can be installed. However, this again is likely to led to substantial extra expense, and is an unlikely solution for existing substations. In addition, this technique is not future proof as the addition of a further transformer at some future time will increase fault levels again.